1. Field of the Invention
The present invention generally relates to a power steering control apparatus for a motor vehicle. More particularly, the invention is concerned with a control apparatus for a motor-driven power steering system which includes an electric motor mechanically coupled to a steering mechanism and electrically connected to a main power supply such as an onboard battery for generating an assist torque for assisting a driver in manipulation of a steering wheel of the motor vehicle.
2. Description of the Related Art
Control apparatuses for the motor-driven power steering system of the type mentioned above are disclosed in, for example, JP-A-H1-267674 (Japanese Unexamined Patent Application Publication No. 267674/1988) and JP-A-H2-249762. In these known power steering control apparatuses, a control apparatus implemented by a single microcomputer is adapted to arithmetically determine an assist torque to be generated by the electric motor as well as the direction in which the motor is to be rotated on the basis of a steering torque applied to a steering wheel by a driver and a vehicle speed of the motor vehicle which are detected by respective sensors.
However, the conventional power steering control apparatuses are not in the position to ensure a high reliability of operation because of possible occurrence of abnormality in the microcomputer and/or peripheral devices thereof, overrun of control software, fixation of output state and/or the like. To cope with this problem, it is conceivable to provide a second or subsidiary processor for arithmetically determining the direction in which the assist torque generating motor is to be rotated on the basis of the steering torque signal indicative of a torque applied to the steering wheel by the driver or a signal obtained through phase compensation of the steering torque signal, wherein the assist torque motor is electrically energized only when the motor driving direction determined by the main processor coincides with the driving direction determined by the subsidiary processor. However, such control system is disadvantageous in that the motor output feedback control performed by the main processor may adversely be affected when the driving direction coincidence/discrepancy decision signal is outputted from the subsidiary processor with a time lag relative to the timing of the torque signal and the direction signal generated by the main processor, because the subsidiary processor is destined to compare the direction signal determined straightforwardly from the steering signal directly inputted to the subsidiary processor and the direction signal determined in precedence by the main processor which includes a phase compensation means for conditioning the output signal from the torque sensor before it undergoes arithmetic operation for determining the assist torque and the driving direction. Under the circumstances, when the steering wheel is rotated in one direction and immediately turned back sharply in the other direction, the assist torque generation by the motor can not follow up the control performed by the control apparatus, possibly incurring outage of the motor and thus uncomfortableness of the steering. This problem may be solved by providing a pair of processors each of high performance so that the time lag mentioned above can be suppressed to a minimum. In that case, the control apparatus will become expensive.
Another problem of the power-steering control apparatus in which two processors are provided for increasing the fail-safe function as compared with the single-processor based control is seen in that when failure or abnormality takes place in the main or subsidiary processor, the steering control operation may fall in the undesirable situation mentioned above in conjunction with the known apparatus.